The present invention relates to a mark sensor device for sensing the brightness of an optical image of an object mark with an image sensor and converting the brightness into an electrical signal.
FIG. 1 schematically shows a conventional mechanism for sensing a mark subjected to image processing, in which mechanism the object mark 1 is required to be positioned at all times in a specified location. To this end, it is necessary to accurately detect the present position of the object mark 1. Conventionally, the position of the object mark 1 has been detected in the following manner: A plane containing the optical image 2 of the object mark 1 is divided by a number of vertical and horizontal lines into small picture elements. Then, the brightness of each picture element is sensed by a two-dimensional image sensor 3. The optical image is then converted into an electrical signal for detection of the position of the object mark 1.
The conventional mark sensing mechanism described above has disadvantages. Specifically, since there are many picture elements, processing of electrical signals from the two-dimensional image sensor 3 involves complicated software and hardware, which in turn results in slow signal processing speed. In addition, this mechanism involves complicated and large-scale hardware as well as a two-dimensional image sensor 3, resulting in a large mark sensing device which is inconvenient in mounting and expensive in cost.